Rotating components such as wheels, crankshafts, drive shafts, gas turbine engine rotors etc, are balanced using various balancing methods which can be considered as either permanent or non permanent. In permanent balancing material is removed from the component to be balanced while in non-permanent methods balanced weights are added to the rotary component.
Rotating components can be balanced by the removal of material from specific balancing lands provided on the component to be balanced, or by the method of mass centering where location features, for example bearing surfaces, are offset machined to compensate for unbalance. In the method of mass centering the location features of the component are machined to their final dimensions such that the position of the axis of rotation, as determined by the location features, is made to be coincident with the center of mass of the component.
There are a number of disadvantages associated with mass centering. One disadvantage is that the component to be balanced is heavier than it would otherwise have to be because of the provision of relatively large balancing lands. Another disadvantage is that slave location features are required to hold the component while the normal location features are machined to their final dimensions. The requirement for relatively large lands and slave location features can add significantly to the weight of the component and this is a significant consideration in gas turbine aero engine applications.
Gas turbine engine rotors have traditionally been balanced using the weight variation present in a set of aerofoil blades to correct not only the disc unbalance but also that of the blades. This has been possible because the rotor blades have traditionally been attachably/detachably mounted in slots provided in the rim of the rotor disc so that the blades can be distributed around the circumference of the disc in such a way that the variations in their weight can be used to compensate for disc and blade unbalance. The net effect of this approach is a balanced bladed disc assembly having zero unbalance.